On a vehicle fitted with a manually operated transmission and clutch system, the driver operates the clutch pedal and selects an appropriate starting gear. An experienced driver will select an appropriate gear which will enable the vehicle to be started smoothly from rest without undue slipping of the clutch and without the subsequent need to make unnecessary gear shifts as the vehicle speed increases. An experienced driver will base the selection of start gear on his mental estimates of the vehicle weight, the vehicle performance, road conditions and gradient. If a gear is selected which is too high for the current weight and gradient then to allow the vehicle to pull away smoothly the driver will slip the clutch until the vehicle speed is high enough to allow complete engagement of the clutch. Clutch slip results in wear on the clutch, the more often the driver slips the clutch the lower the clutch life.
A conventional automatic transmission is normally a automatic transmission uses a torque converter and an epicyclic gear train. An automated mechanical transmission (AMT) is normally considered to be a stepped gear transmission and will have a clutch device between it and the engine to selectively connect the engine to the transmission. The clutch device is normally a friction type device, rather than a torque converter. An AMT normally includes means for controlling the selection of gear ratio. Operation of the clutch can be by pedal or by an automated means. An AMT is normally provided with a manual override control for selecting or holding a ratio. With an AMT the selection of a start gear can be controlled to give optimum start performance and minimise clutch slip time.
Whilst clutch slip can be minimised by always starting in 1st gear, such an implementation would often result in a large number of unnecessary gear shifts and would be tedious and unacceptable for the driver. Fuel efficiency would also be adversely affected.
On current AMT implementations, the selection of a starting gear is still made by the driver by operation of a selector switch. Such a switch will normally be in the form of a selector lever mounted on the steering column or beside it, or a dash panel mounted switch and is also used for gear selection or overriding the gear selected/determined to be appropriate by the CPU. These switches can allow the driver to select a less than optimum gear for starting and so possibly reduce clutch life. Often an AMT will have a pre-programmed start gear which may be pre-programmed into the control system of the AMT. Alternatively, and frequently, it may be set at the beginning of every journey or even re-set during the journey. A disadvantage of such systems is that they are relatively inflexible in that they do not take account of changes in road conditions or vehicle weight due to a loading or unloading stop.
It is possible on some applications of the current AMT systems to use a central processing unit (CPU) to calculate (or measure using sensors, which supply information to the CPU) the gradient of the road, the gross vehicle weight (GVW). Using these values of gradient and vehicle weight it is possible to calculate an optimum starting gear for the system in the prevailing conditions and select this independently of the driver. The benefits of this system are to protect the clutch and transmission from starting in too high or too low a gear for the current conditions, so allowing a longer clutch life and reduced abuse of the vehicle drive train.
The system is also simplified for the driver. He does not need to select a starting gear, although an override system may be included if it is considered appropriate.